2.1 Half Duplex with One-Wire

With half-duplex communication in a one-wire system, every device connected to the same line will need to be able to change the state of the line. The state of the line can either be High or Low. When the line is high, the measured voltage is usually the same as the supply voltage V_CC. When the line is low, it will usually be pulled to ground, and zero volts will be measured on the input.

When there are no devices communicating, the line will be in a predefined state, usually High. When one device wants to send data, it will have to pull the line low. As all devices are on the same line, they cannot all send data at the same time. The devices will have to share the line through some form of time multiplexing.

Because all devices can pull the line low when the line is high, a special output circuit is needed to protect against potential short circuits. The following chapter will discuss a solution for this, called open-drain.